This invention relates to a work holder of the type used in sewing machines that form two-dimensional stitch patterns on a series of workpieces, one after another. In particular, the invention relates to a work holder that has easily changeable clamping means that are firmly held in place while being used to clamp a workpiece in place to form a pattern of stitches therein and that remain firmly held in place for as many repetitions of the sewing operation as may be desired but that can be easily removed and replaced by other clamping means to clamp workpieces of a different size.
One of the common repetitive functions in the sewing industry is to sew a label on a panel of material and to repeat that operation over and over as quickly as possible to produce a large number of identical, finished workpieces. The labels are stitched. to the larger panel by four rows of stitches formed by moving the panel about on the bed of the sewing machine while the label is held in a fixed position on the panel. In order to do all this as rapidly as possible and to place the stitches exactly where they should be, the movement of the panel is not directly controlled by a human operator. Instead, sewing machines of the type normally used for this purpose, such as the Mitsubishi Model 0303 and Model 0604, have a cloth plate that rests on the bed of the machine and is connected to a mechanism, known as the X-Y mechanism, that moves the cloth plate in a series of incremental steps in two perpendicular directions. These incremental movements are controlled by a digitally programmed electronic controller, which is capable of causing the cloth plate to move not only in a rectangular pattern to set a label but in much more complex patterns to form letters, numbers, or artistic emblems. All of the movement required to form the pattern of stitches is carried out by the cloth plate; the needle moves reciprocatingly in a fixed path that intersects the panel.
The panel of material is clamped against the surface of the cloth plate by a clamp frame that is part of a work holder, which also includes a base and an arm supported by the base. The base is rigidly attached to the cloth plate and to the X-Y mechanism to move in unison therewith according to the stitch pattern to be formed. The arm extends out from the base, and the clamp frame is supported at the free end of the arm. Means are provided to move the clamp frame toward the cloth plate with sufficient firmness to clamp the panel of material against the cloth plate to cause the panel of material to move precisely in unison with the cloth plate according to the recorded stitch pattern.
A typical clamp frame consists of four sides surrounding a rectangular open area of the same size and shape as the label to be sewn to the panel. However, it is not enough merely to place a label in the open area and expect it to lie therein as in the surrounding sides of the clamp frame were a nest; the label must be pressed against the underlying panel of material so that it will remain in a fixed location as the panel moves in accordance with the stitch pattern. Furthermore, as in the sewing of any material, the label must be held down in the region adjacent the needle to prevent tenting. This may be done by means of a hollow cylindrical presser foot that surrounds the needle, or it can be done by a plate that is nearly as large as the open area of the clamp frame and is pressed down against the central part of the label by a separate actuator. This leaves a narrow gap between the perimeter of the plate and the inner perimeter of the frame, and the stitches to sew the label to the underlying panel of material can be formed in that gap.
The plate that presses against the central part of the label is supported by a shaft that extends across the gap and would prevent stitches from being formed directly under it. However, if stitches are to be formed in a closed rectangular pattern, the shaft that supports the plate has an eccentric section directly over the location of the gap, and means are provided to pivot the shaft about 180.degree. to throw the eccentric first to one side and then the other so that stitches can be formed in the region that would be covered by a straight shaft having no eccentric.
One of the problems encountered by the users of pattern stitching machines, especially in attaching labels to panels, is that labels come in different sizes, and each size requires a clamp frame that just fits it. Clamp frames of the type used heretofore have been attached by machine screws to a clamp foot that is moved up and down to release and grip the panel of material onto which the label is to be sewn. When labels of a different size are to be sewn by the machine, the operator, or a mechanic, must unscrew the machine screws, remove the clamp frame, align a different clamp frame with the clamp foot, and reinsert and tighten the screws. During all of that time, the machine is inoperative, and if such changes have to made often, the cumulative down-time can make the resulting sewn products excessively expensive and non-competitive.